A secure communication device for high-speed encryption/decryption authentication including network stack processing. An encryption/decryption authentication controller of the secure communication device acquires the result of processing of the encrypted/decrypted or authenticated previous packet from an encryption/decryption authenticator and controls a network protocol processor so that the second half of the processing of the network protocols of the previous packet and the first half of the processing of the network protocols of the current packet are continuously performed.